So called "standing waves" appear on a transmission line which is not terminated in its characteristic impedance; i.e. when the power of an RF signal applied to the transmission line from a source (such as a transmitter) is not fully absorbed by the load (such as an antenna).
The reflected voltage has always a 180.degree. degree lead over the reflected current. The vectorial sum of the incident and reflected voltage or current represent their r.m.s. value at any point along the line.
This phenomena is usually defined in terms of a "Standing Wave Ratio" (SWR) based upon the ratio of the reflected power over the transmitted power according to the formula: ##EQU1##
Methods for the directional measurement of power on transmission lines are well known to those skilled in the art (see "An Inside Picture of Directional Wattmeters" by Warren B. Bruene, QST Magazine, April 1959 issued page 24).
In the prior art, Standing Wave Ratio meters required two totally independent measurements. The first measurement would yield only the reflected power; while the second yield only the transmitted power. Separate analog panel meters would display the two resulting measurements which had then to be correlated in order to obtain the power ratio necessary to compute the SWR. Another commonly employed method for SWR measurement consists in calibrating a meter to display a full-scale reading for the incident power then switching the meter to read the reflected power. This sequential method is based on the assumption, which is not always correct, that the signal on the transmission line remains constant throughout the two measurements. These instruments were relatively of simple design and inexpensive, since usually there is no requirement for high accuracy in the tuning of a transmission circuit.
Ratiometric techniques which would give a direct reading have seldom been employed in the past in the design of SWR meters due to the unwarranted cost increase and added complexity which they would entail.
The appearance on the market of low-cost large scale integrated circuit modules has opened new opportunities for the design of inexpensive yet more advanced SWR meters.